The Basic Biology of Bitter Taste. The perception of bitter taste is thought to have evolved as a mechanism to protect against the ingestion of toxic materials, and is the result of ligand-activation of one of more than 25 different bitter taste receptors, so-called T2Rs (1). These receptors are found on the tongue in what are called fungiform papillae, mushroom-like structures that contain taste buds with receptors responding to a variety of tastes including sweet, salty, sour, umami, and bitter. A prototypical example of one of these bitter ligands is phenylthiocarbamide (PTC), which actives the T2R38 receptor. While initially identified in type II taste cells, T2R38 is also expressed in nasal epithelium, where it participates in innate immune defense responses to invading bacteria (2-6).